A large part of the material that makes up the Moon came from the early Earth, reports a study published online this week in Nature Geoscience. The findings are in contradiction to numerical models of the formation of the Moon by the impact of a Mars-sized object with the early Earth.

Junjun Zhang and colleagues compared the isotopic signature of titanium in lunar and terrestrial samples, and corrected the lunar signature for secondary alterations associated with exposure to cosmic rays. They found that the isotopic titanium signature of the Earth is identical to that of the Moon within about four parts per million. Because the Mars-sized impactor is expected to have been isotopically different, the measurements suggest that the Moon is either made of material from Earth’s mantle, or that intense mixing occurred after the impact.

In an accompanying News and Views article, Matthias Meier suggests that this finding “shifts the focus of future work on the giant impact towards the later part of the story.”